A system for controlling a load current to a load, wherein the load current is based on the automatic detection of a normal or abnormal load condition, has been heretofore known. FIGS. 5 and 6 are, respectively, a circuit diagram and a block diagram showing the configuration of a system of this kind, which has been proposed by an Applicant of this invention in U.S. patent application No. 07/282,416.
The load condition detecting and driving system of FIGS. 5 and 6 is comprised of a load driving means A, a current detecting means B, a discriminating means C, a holding means D, an indicating means E, and a holder releasing means F. The voltage of a battery 6 is applied to the load driving means A, and current is supplied to load 9 through current detecting means B, connected between the load driving means A and the load 9.
As shown in FIG. 5, the load driving means A includes transistors 4 and 7, constituting a switch circuit. When driving signal V.sub.IN is applied to load driving means A, transistors 4 and 7 are rendered conductive, and the load current flows from battery 6 through transistor 7 and through current detecting resistor 8 of the current detecting means B to load 9.
As current flows to load 9, since both ends of the current detecting resistor 8 are respectively connected to the input terminals of differential amplifier 10 of the discriminating means C, a voltage V.sub.O corresponding to the load current flowing through the current detecting resistor 8 is obtained at the output terminal of differential amplifier 10.
This voltage V.sub.O is compared with a threshold voltage V.sub.3, which is set by dividing the voltage V.sub.B supplied from battery 6 using series resistors R.sub.6-1 and R.sub.6-2, by means of differential amplifier 11. Similarly, voltage V.sub.O is also compared with another threshold voltage V.sub.4, which is set by dividing the voltage V.sub.B using series resistors R.sub.7-1 and R.sub.7-2, by means of another differential amplifier 12.
When the load 9 is in an abnormal short circuit condition, the load current is greatly increased, and the voltage V.sub.O outputted from differential amplifier 10 becomes larger so that V.sub.O exceeds V.sub.3, as shown in FIGS. 7(2) and 8. Thereafter, the resulting HIGH output signal of amplifier 11 is applied to the input of holding means D, which latches this state and energizes indicating means E, causing light emitting diode (LED) 19 to illuminate and indicate a short circuit abnormal condition.
When the load is in an abnormal open circuit condition, drawing essentially no load current, the voltage V.sub.O outputted by differential amplifier 10 will be very low so that V.sub.O is less than V.sub.4, as shown in FIGS. 7(3) and 8. Thereafter, the resulting HIGH output signal of differential amplifier 12 is applied to the input of the holding means D, which latches this state and energizes indicating means E, causing another light in the LED 20 to illuminate and indicate an open circuit abnormal condition.
In either the case of the short circuit condition or the open circuit condition of the load, the application of the driving signal V.sub.IN to the load driving means A is interrupted by the use of logic means and the HIGH output signal from the holding means D. Consequently, the system is placed in a non-driving state, and the abnormal load condition is identified.
When the load is in a normal condition, voltage V.sub.O is between V.sub.3 and V.sub.4, and normal operating current is supplied to the load.
In the above-described system, even when the load is in an acceptable condition, the load condition may vary significantly from a normal condition. Hence, the load current is also changed in response thereto, causing the voltage V.sub.2 to change. If the load condition varies sufficiently, despite that it is not either in the open circuit condition or the short circuit condition, the circuit of FIGS. 5 and 6 may misjudge the load to be in an abnormal condition and undesirably discontinue current to the load.
Further, since the divided voltage V.sub.B from battery 6 forms the threshold voltages V.sub.3 and V.sub.4, when the voltage V.sub.B of battery 6 drops for any reason, the threshold voltages V.sub.3 and V.sub.4 are changed, and the discriminating standard is changed. Therefore, an accurate discrimination between a normal operation condition and an abnormal operating condition becomes impossible.
Moreover, if the load 9 is in a short circuit condition, a predetermined amount of time is required for the discriminating means C to discriminate this abnormal condition and cause the signal from the holding means D to interrupt the input of the driving signal V.sub.IN to the load driving means A and turn off transistors 4 and 7. Thus, the switching off of transistors 4 and 7 is not instantaneous after load 9 is in a short circuit condition. As a result, the high current supplied to the short circuited load for this predetermined amount of time can possibly cause damage to the short circuited load or other circuits. In addition, using the system of FIGS. 5 and 6 makes it difficult to determine the condition of the load if a driving signal V.sub.IN is not already applied to the load driving means A.